This invention relates to geophysical prospecting, and more particularly relates to geophysical prospecting by detection of acoustic responses generated by an electric field.
Conventional seismic prospecting techniques for land involve the use of an appropriate source to generate seismic energy and a set of receivers spread out along or near the earth's surface to detect any seismic signals due to seismic energy being reflected from subsurface geologic boundaries. These signals are recorded as a function of time, and subsequent processing of these signals, i.e. seismic data, is designed to reconstruct an appropriate image of the subsurface. In simplistic terms, this conventional process has seismic energy traveling down into the earth, reflecting from a particular geologic layer (impedance contrast), and returning to the receiver as a reflected seismic wave.
The seismic energy may be so-called shear waves (S-waves) or so-called compressional waves (P-waves). Shear waves and compressional waves differ with respect to their velocities, angles of reflection, vibrational directions, and to some extent the types of information that may be obtained from their respective types of seismic data. However, both types of waves suffer similar attenuation by earth formations; that is, the earth formations tend to attenuate the higher frequency components and allow the lower frequency components to pass through the earth relatively unattenuated. This means that for deeper formations the low frequency content of the reflected seismic energy contains the information about the underlying subsurface formations. However, because of the low frequency of the detected reflected seismic energy, the resolution of the reflected seismic energy may be insufficient to allow for detection of very thin geologic layers.
Further, if the impedance contrast between adjacent but distinct geologic layers is very small, very little seismic energy is reflected and the distinctness of the geologic layers may not be discernable from the detected or recorded seismic data. Thus, efforts continue to be made to appropriately image subsurface layers by geophysical prospecting techniques other than by seismic prospecting techniques alone.
A new technique for geophysical prospecting is described in U.S. Pat. No. 4,904,942 to A. H. Thompson issued Feb. 27, 1990. This patent describes a method for petroleum exploration involving generating an electric field in a fluid-containing porous earth formation with a seismic wave. More particularly, the pressure gradient associated with a seismic wave causes fluid to flow in the pores of a porous rock formation. When the fluid contains charged species, the flow produces a distortion of electric dipole layers and thereby an electric field. The resulting electric field propagates to the surface of the earth where it may be detected with electromagnetic sensors.
This so-called electroseismic prospecting technique may provide additional information about subsurface geologic layers, but will be most sensitive to high permeability fluid-filled layers. This electroseismic technique is not sensitive to low permeability shales or shaly rocks. Thus, this technique will not be able to determine whether or not the detected, porous proposed reservoir layer is capped by a low permeability sealing layer, or what low permeability layer may be the source of the petroleum fluids, if any, in the proposed reservoir layer.
In addition, there have been published articles discussing the theory and use of electrokinetic effects. In general, these articles describe laboratory devices designed to convert electrical energy into acoustic or electromechanical energy. For example, one such device is a micropipette apparatus employing a porous material containing fluid where an applied electric field causes the porous material to eject a very small quantity of the fluid. Such laboratory devices find limited applicability due to their low efficiencies compared to conventional loudspeakers and pumps.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and improved methods and apparatus are provided for geophysical prospecting.